1. FIELD OF THE INVENTION
The invention relates to the production of a bath of molten metal or alloys wherein liquid nitrogen or argon is discharged throughout the process of the production of molten metal or alloys and more particularly to a lance for discharging the said liquid gas.
2. PRIOR ART
It is known from British Patent 987 190 to cast continuously a molten metal from a ladle into an ingot mould and to shield the jet of molten metal with a solidified or liquefied inert gas such as liquid nitrogen (when the presence of this element in the metal is not harmful) or argon along with the surface of molten metal in said ladle to avoid oxygen, hydrogen and nitrogen pick-up from the surrounding atmosphere.
In electrical furnaces, milten metal comes from the heating up of pieces of metal or of scrap metal which are progressively melted in said furnace, while new pieces of metal or scrap metal are added throughout the melting phase.
Almost any open face surface of hot solid or molten metal can be protected against oxygen, hydrogen and/or nitrogen pick-up by injection of liquid argon or nitrogen (if nitrogen pick-up is not a problem) above the said surface. Said process makes it possible to prevent contamination from not only atmospheric oxygen but also from humidity generating hydrogen in the melt or from nitrogen in as much as liquid nitrogen is not used.
Furthermore, it is possible with such a process to protect the pieces of scrap metal or new stocks of metal in the stage of pre-heating above the liquid bath of molten metal prior to melting. The atmosphere above the metal is selected according to the nature of metals, alloyed metals, alloys or pure metals and it must be maintained above and around the elements of the charge throughout the whole melting and holding operations, from the very moment the charge begins to heat up, up to the moment the metal is tapped.
Contrary to the shielding of the surface of the metal, preheated, solid or liquid with argon or nitrogen in gaseous state, where the injection velocity of said gases creates turbulence and hence an ingress of atmospheric air diluting the inert atmosphere, protection of the surface of the metal surface with liquefied gas makes it possible for said liquefied gases to reach the bottom of the furnace or the surface of the molten metal : they first vaporize as cold heavy gases (which are heavier than the same at room temperature) which in turn, heat-up, expand and flush out all the atmospheric air in the furnace.
However, there are some limitations to this protection against hydrogen, nitrogen and/or oxygen pick-ups.
When the pieces of metal are partly covered by water, this water can come into contact with the molten bath and generate hydrogen bubbles in the bath along with some metal oxides. Hydrogen can also be generated by the flames of the burners, if any are used to heat the molten metal. Oxygen can be generated from deeply oxidized metal scrap introduced in the bath and nitrogen can be generated from nitrogen in the alloying elements namely in arc furnaces in the region of electrodes.
When, according to the process disclosed in the copending application Ser. No. 077168 filed on July, 24, 1987 U.S. Pat. No. 4,806,156, liquid nitrogen or liquid argon is poured into the furnace during the production of molten metal, it is necessary for the level of diphasic argon or nitrogen to be as low as possible : the inventors discovered during their experiments that the presence of nitrogen or argon gas in the lance used to deliver the liquid gas generates turbulences in said lance and thus some splashes occured in the molten metal which could be very dangerous for people present in the vicinity of the furnace. It also destroys the inert atmosphere due to the pulsating flow, which provides non maintenance of liquid in the furnace or on the metal surface and an ingress of air due to gas velocity.